Foam is an important property of cleaning compositions, especially for personal care compositions, such as hair shampoos, body gels, dentifrices, and shaving creams. Esthetically, foam is an important property because consumers equate a rich, long-lasting foam to a high quality product that works well. Functionally, foam is an important property because the foam first acts as a carrier to deliver cleaning surfactants to the skin or hair, then acts as a carrier to help remove emulsified soil and sebum from the cleaned skin or hair.
However, to achieve these esthetic and functional goals, the foam must be present in a wet, or spherical, form. If the generated foam is unstable, the foam changes from the wet form to the dry, or hexagonal, form relatively quickly, i.e., in less than about two minutes. Dry foam bubbles break quickly, and, therefore, for dermatogically preferred compositions do not provide the esthetic and functional foam properties typically required for consumer acceptance of the composition.
Foam is created by dispersing air or a gas in a surfactant-containing liquid. The mechanism of dispersing a gas in a surfactant-containing liquid is similar to the dispersion of two immiscible liquids during formation of an emulsion. Consequently, gas bubbles dispersed in a liquid are stabilized in the same manner as emulsions, i.e., by formation of surfactant layers at the gas-liquid interface. The surfactant layers keep the gas bubbles separated and prevent “coalescence,” i.e., the merging of small gas bubbles to form larger gas bubbles. In general, more dense and more compact surfactant layers form smaller bubbles and retard the coalescence mechanism.
It is well known that because of the very large density difference between the dispersed gas and the liquid, the gas bubbles rise to the top of the liquid. The enriched concentration of gas bubbles at the top of the liquid appear as “foam.” Initially, all of the gas bubbles in the foam are spherical, there is sufficient space between each individual spherical gas bubble for the presence of the surfactant-containing liquid, and the foam behaves like an emulsion. Such a foam is termed a “wet foam.”
Over time, the liquid present in the interstices between the individual gas bubbles drains out due to gravity. Depending on the nature and chemical structure of the surfactant in the liquid, lamellar liquid crystalline layers form and arrange at the gas-liquid interface. If the lamellar layers have a low viscosity, the surfactant-containing liquid between individual gas bubbles can drain relatively easily, and the spherical form of the foam bubbles can change into a hexagonal form relatively quickly. Hexagonal bubbles quickly break. The transition of a foam from the spherical form to the hexagonal form due to foam aging can be observed visually. Foam in the hexagonal form is termed a “dry foam.” Dry foams are unstable, which leads to a rapid reduction in foam volume due to rapidly breaking bubbles.
However, if the lamellar surfactant layers have a relatively higher viscosity, the transition from a spherical foam to the hexagonal form can be delayed. The speed of the transition of a foam from the spherical to hexagonal form determines how the foam is used in practical applications, and also determines how the foam is perceived esthetically. For example, for shampoos and shower gels, foams having a foam transition of about two minutes or less, i.e., a metastable foam structure, is desirable. Foam stability is controlled by many factors associated with the physiochemical properties of the surfactant solution such as surface tension, film surface tension (lamellar foams), surface viscosity and elasticity amongst others. A more stable foam could lead to insufficient wetting and distribution of the surfactant on the skin or hair, because during application of the shampoo or shower gel to the skin or hair, some parts of that foam undergo the transition to hexagonal state that allows the surfactant to drain from between the bubbles to contact and wet the hair or skin. Then, by continual rubbing of the shampoo or shower gel on the skin or hair, new foam bubbles are generated, which act as a carrier to lift and remove soil and sebum from the skin or hair and additional water rinses it off. Esthetically, spherical foam is desired. Functionally, the transition to the hexagonal form and regeneration of the spherical foam provides cleansing.
Other foam applications, e.g., shaving foams, require a foam having a much greater stability because the applied foam is not regenerated by continual rubbing techniques and because of the relatively long time required to complete the entire shaving operation. Furthermore, a controlled and sufficient wetting of the hair and skin is required for a smooth shaving operation. Sufficient wetting when a surfactant is foamed occurs only if the surfactant-containing liquid can drain from the foam lamellae to contact the skin, and drainage occurs more readily when the foam bubbles are in the spherical form.
The difference in structure between a wet, spherical foam and a dry, hexagonal foam is illustrated in FIGS. 3 and 4 of U.S. Pat. No. 5,911,811. FIG. 3 of U.S. Pat. No. 5,911,811 clearly shows both the lamellar liquid crystalline surfactant structure that stabilizes each bubble of a wet foam and the large amount of surfactant-containing liquid between individual bubbles. The relatively thick surfactant structure illustrated in FIG. 3 also retards the coalescence of neighboring bubbles into a single, larger bubble. In contrast, FIG. 4 of U.S. Pat. No. 5,911,811 shows a lack of a stabilizing surfactant structure around the hexagonal bubbles and the relative absence of surfactant-containing liquid between the bubbles.
The most commonly used sulfated anionic surfactants are well known for providing a high volume of a stable foam and having an excellent ability to emulsify soils and oils, i.e., to act as an efficient cleaner of skin and hair. Sulfated anionic surfactants include the anionic sulfates and the anionic sulfonates. As a result of these properties, sulfated anionic surfactants have been the primary surfactant used in shampoos and other skin and hair cleaning products. However, sulfated anionic surfactants have disadvantages. For example, the sulfated anionic surfactants strip the hair of natural oils that condition the hair and thereby can damage the hair and give freshly shampooed hair a dry feel. Sulfated anionic surfactants also are harsh to the skin and eyes, and thus sulfated anionic surfactants are generally unsuitable for use in baby shampoos and pet products.
Amphoteric and nonionic surfactants are relatively mild to the skin and eyes and do not strip the hair of natural oils. However, amphoteric and nonionic surfactants typically generate a poor foam in comparison to a sulfated anionic surfactant. Therefore, shampoos and similar cleaners based primarily on amphoteric and nonionic surfactants have not achieved good consumer acceptance. But, amphoteric and nonionic surfactants have been used in conjunction with sulfated anionic surfactants in attempts to provide a shampoo that takes advantage of the foaming properties of a sulfated anionic surfactant, while tempering the disadvantageous properties of the sulfated anionic surfactant with a nonionic or amphoteric surfactant.